The present invention pertains to Schottky diodes, and more particularly to Schottky diodes fabricated using InP semiconductor material.
Schottky diodes are well-known in GaAs and Si technologies. Some Si-based Schottky diodes have fairly low turn-on voltages but suffer from high series resistance and relatively poor frequency response. Si-based Schottky diodes are not easily integrated into InP-based semiconductor fabrication.
Low turn-on voltages are often desirable for Schottky diodes, particularly for mixer applications. U.S. Pat. No. 5,665,999 issued Sep. 9, 1997 to H. Brugger, teaches that low turn-on voltage Schottky diodes permit a low conversion loss without requiring a high pumping capacity in the local oscillator, and without using a bias voltage which may lead to Townsend current-hum disturbances. A GaAs Schottky device with reduced turn-on voltage is taught as fabricated using a graded InxGa(1xe2x88x92x)As layer over a GaAs substrate, where the In content, x, increases continuously in the direction of the metal contact. Compatibility with InP is not taught, and the spatially varying lattice mismatch created by this process is thought potentially unreliable with InP technologies.
Schottky diodes compatible with InP semiconductor technology are known. For example, U.S. Pat. No. 5,652,435, issued Jul. 29, 1997 to E. Martin et al., describes a Schottky diode optical detector wherein the light-sensitive InGaAs layer is cladded front and back with InAlAs current-blocking layers. This device has a relatively high turn-on voltage similar to that of conventional GaAs Schottky diodes.
Some investigation into the relationship between Schottky barrier height and the proportion of In in InxAl1xe2x88x92xAs material on an InP substrate has been undertaken by C. L. Lin, et al. in xe2x80x9cComposition dependence of Au/InxAl1xe2x88x92xAs Schottky barrier heights,xe2x80x9d Applied Physics Letters 49 (23), Dec. 8, 1986, pp. 1593-1595. Lin performs tests on the noted compound for 0.45xe2x89xa6xxe2x89xa60.55, a range producing relatively high Schottky barriers.
InP-compatible Schottky diodes using a graded superlattice Schottky layer are also known. For example, U.S. Pat. No. 5,198,682 issued Mar. 30, 1993 to C.-S. Wu et al. describes an infrared photodetector using a superlattice having a graded dopant concentration, thereby developing an internal field which aids in the collection of photoexcited carriers. Lee et al., in Appl. Phys. Lett. Vol. 54, May 1989, pp. 1863-1865, teaches use of a graded superlattice of InGaAs/InAlAs to obtain enhanced Schottky barrier height. U.S. Pat. No. 5,572,043, issued Nov. 5, 1996 to Shimizu et al. describes another SL-based Schottky diode, using sublayers of variously lattice-mismatched compounds and alternating compressively and tensilely strained layers to balance the mismatches. Each of these three references describe Schottky diodes having a relatively high turn-on voltage of xcx9c0.6V-0.7V.
InP-compatible Schottky diodes having high turn-on voltages are known. For example, U.S. Pat. No. 4,471,367 issued Sep. 11, 1984 to C. Chen et al. discloses a MESFET-gate Schottky structure composed of a thin, heavily doped InGaAs layer overlying a layer of low-doped InGaAs having an increased barrier height. Another example is U.S. Pat. No. 4,954,851, issued Sep. 4, 1990 to W. Chan, which describes a Schottky diode with a cadmium-containing layer overlying an InAlAs layer. The Cd-containing layer enhances the barrier height of the diode.
Thus, Schottky diodes with reduced turn-on voltages exist but are not known to be compatible with InP-based fabrication techniques. Schottky diodes compatible with InP are known, but all have conventional or high turn-on voltages. Yet, InP-based fabrication technologies are desirable, and low turn-on voltage Schottky diodes are particularly useful in certain applications, such as high-frequency mixers. A need therefore exists for low turn-on voltage Schottky diodes, and a method of making the same, which are compatible with InP fabrication technologies.
The present invention addresses the need for low turn-on voltage Schottky diodes compatible with InP-based fabrication by employing an InP lattice-compatible Schottky layer which interfaces with the Schottky metal layer and provides a relatively low Schottky barrier height resulting in a low turn-on voltage.
The Schottky metal layer is preferably Ti/Pt/Au. The Schottky layer is preferably grown over a low-doped GaInAs layer, which in turn is preferably grown over a highly doped GaInAs layer. The highly-doped GaInAs layer forms the Schottky cathode contact. The cathode contact is grown over a buffer layer, such as 2500 A of AlInAs or 100 A of undoped GaInAs, preferably grown directly on the InP substrate.
According to the present invention, the Schottky layer is processed to produce a low turn-on voltage.
According to a first embodiment of the invention, the Schottky layer includes strained Al(1xe2x88x92x)InxAs, and for x=0.70 produces a turn-on voltage of 0.25V @ 1 mA. The corresponding 0.3 Al content is lower than that which lattice-matches InP, and accordingly the thickness of the layer is preferably around 100 xc3x85 to limit effects from the strain.
According to second, third and fourth embodiments of the invention, the Schottky layer includes a chirped superlattice composed of a plurality of periods, each period containing sublayers of Ga0.47In0.53As and Al0.48In0.52As, which are lattice-matched to InP. The proportional thickness of the two compounds is varied from period to period to provide a step-wise, or chirped, grade in composition of the superlattice from GaInAs toward AlInAs. The turn-on voltage can be precisely controlled by truncating the grading steps before completing a transition to predominantly AlInAs.
Throughout this patent, GaInAs refers to Ga0.47In0.53As, and AlInAs refers to Al0.48In0.52As, unless a different composition is specified. In one aspect, the present invention relates to a Schottky diode, and a method of making the same, which is fabricated on InP material and employs a Schottky layer including InxAl1xe2x88x92xAs with x greater than 0.6, or else including a chirped graded superlattice in which successive periods of the superlattice contain progressively less GaInAs and progressively more AlInAs, the increase in AlInAs being terminated before the proportion of AlInAs within the last period (adjacent the anode metal) exceeds 80%. Such fabrication creates an InP-based Schottky diode having a low turn-on voltage which may be predictably set within a range by adjusting the fabrication parameters.